Gamma correction is well known in the art to correct video intensity values for causing a linear range of video intensity values to be displayed on various displays. The video data obtained by photoelectric transfer and having a non-linear characteristic are corrected in accordance with a pre-given correction curve, the so-called gamma correction characteristic, so that the image is displayed as a linear gradation of intensities.
As typical video displays, for example cathode ray tubes (CRTs), have a non-linear transfer characteristics, i.e. the reproduced brightness on the CRT screen is a non-linear “gamma” function of the controlled-grid video drive (e.g. gamma=2.2). To achieve a linear gradation of intensities, the video signal at the TV transmitter side is pre-corrected (e.g. gamma=0.45) for a standard CRT display. When displayed graphics are produced by a workstation or when images are recorded by a camera etc., it is necessary to compensate for this non-linearity. To produce a linear gradation of intensities, this compensation is called gamma pre-correction. The CRT display performs the gamma post-correction, and these pre-and post corrections together make approximately a linear transfer function (gamma=1.0).
If the display plus receiver electronics differ substantially from the reference display in the studio, we need to apply some extra gamma correction. Another reason for wanting extra gamma correction is that the optimum value of the overall gamma depends on ambient light conditions: a linear transfer function (gamma=1.0) is not always best. It is the subject of this invention to provide a method for this extra gamma correction.
Different transmission standards may prescribe different gamma pre-corrections, and the type of display has a strong impact on the gamma post-correction characteristic, particularly on the value of the gamma correction factor. This correction factor of a CRT display depends among others on the output impedance of the video output amplifiers. Further it should be kept in mind that the broadcast images have been produced to look good on a studio monitor, thus it is the gamma correction factor of that studio monitor that actually defines the system value of the gamma pre-correction factor. Further, there are other display types that have a totally different transfer function, like PDP and DLP or the LCD-display. In all these cases, an extra gamma correction function must comply to these transfer functions to properly correct the distortion of the display.
A simple method of performing gamma correction on digitized intensity signals is known which translates each of the incoming n-bit red, green and blue color intensity values to compensated n-bit color intensity values using a color lookup table for each color. The lookup table is typically stored in a solid state memory and includes a range of color intensity values each of which is associated with the corresponding gamma-corrected intensity value. The gamma-corrected value is derived from the gamma correction characteristic and stored in the lookup tables. The gamma-corrected values read from the table may be converted into analog intensity signals that are displayed. This known solution is however, relatively inaccurate, resulting in a series of problems with having enough resolution in the blacks, particularly if the display requires gamma correction with a high gamma value.
EP 0 457 522 shows a circuit including a linear interpolator for performing a required gamma correction characteristic. This implementation approximates the gamma correction characteristic by line segments of a polygonal line, each of the line segments being a section of a straight line which is defined by slope data of the correction function at the input intensity and the intercept data of the straight line intercepting the output level axis Y at input level X=0%. By this known circuit, one lookup table stores the slope data, the second lookup table stores the intercept data, and the slope data multiplied by the input intensity is added to the intercept data to represent the output intensity. This known implementation is further optimized in EP 0 457 522 for gamma<1 by storing more data in the region near black where the differential gain is>>1. The problem of having enough resolution in the blacks, if the display needs full gamma correction with gamma>1 is, however, not addressed.